I've never used a tach to set my needles. I normally start by getting both needles close, but defintely rich. I then adjust the low needle a lean as possible a still get good acceleration when I jam the stick. The low needle will lean more, but the engine will gag on acceleration. I then lean the high needle until I find the lean drop-off point, then richen it back about 1/4-3/8 a turn. It took longer to type this than actually do it! I don't claim to be an expert, but I haven't burnt or overheated a gas burner in 15 year of flying them either..